Europe’s push toward a carbon‑free power system hinges on whether renewable energy can reliably meet demand when the wind stops blowing or the sun disappears for days. Short‑duration batteries, dominated by lithium‑ion chemistry, typically deliver power for up to 4–8 hours and are excellent for smoothing minute‑to‑hour fluctuations. They cannot, however, bridge multi‑day wind lulls without calling on gas‑fired peaker plants or curtailing surplus renewables. It’s no surprise that long-duration storage of energy (LDES) represents a critical technology. LDES systems, storing energy for 8 hours or more, can shift entire days of generation, reduce renewable curtailment, and reduce the need to over‑build wind and solar.
Until recently, options for multi‑day storage or long-duration storage were either geographically constrained (pumped hydro) or expensive and unproven at scale. That began to change when companies like the Dutch start‑up Ore Energy worked with abandoned technology from the 20th century for storing energy. But the Dutch company went one step further and has now connected the world’s first grid‑connected iron-air battery system, in Delft.
In this article I will – literally – explain the chemistry behind iron-air batteries, why long-duration storage matters, and I will also compare iron‑air with other emerging storage technologies.
Ore Energy’s Iron-Air Batteries Breakthrough
How The Technology Works
Iron-air batteries store energy by exploiting the reversible rusting of iron. When the battery discharges, metallic iron reacts with oxygen from the air to form iron oxide (rust), releasing electrons and generating electricity. During charging, an external power source reverses the reaction, converting iron oxide back into metallic iron. The electrolyte is water‑based and alkaline, and the cathode is a porous air‑breathing electrode that allows oxygen diffusion. Because both iron and air are abundant and the reactions occur at low voltages, the system is inherently safe and does not suffer from thermal runaway.
The basic cell was studied in the 1960s but shelved due to a lack of market demand for long-duration storage back then.
Ore Energy is one of many companies who revived the concept, bit Ore Energy also optimized it for grid applications. Its cells are assembled into modular 40‑foot containers; each full‑scale container is designed to deliver up to 4.2 MWh of storage capacity and sustain 100 hours (over four days) of continuous discharge. Because the system uses iron, air and water, it can operate without scarce materials such as lithium or cobalt, which are increasingly subject to supply constraints and geopolitical risk.
CEO Aytaç Yilmaz claims the technology is seven to ten times cheaper than lithium‑ion on a per‑kilowatt‑hour basis, though full cost data have not yet been published. The open architecture also allows maintenance and recycling of the metallic iron, potentially extending system life.
100 hours of battery storage without any rare minerals. Just iron and oxygen, rusting and de-rusting according to the fluctuations in renewable energy generation. Developed at @tudelft! https://t.co/ulTWTelMpi
— Maarten Boudry (@mboudry) August 4, 2025
Iron-Air Battery Pilot project at The Green Village
On 30 July 2025, Ore Energy announced that it had connected its first iron-air battery to the electrical grid at The Green Village, a living lab at Delft University of Technology. This unit, with a capacity of less than 1 MWh, is the first iron-air system to be grid‑connected anywhere and the first long-duration storage device designed, built and installed entirely within the European Union. The pilot has been operating since mid‑May 2025 and will be monitored over six to twelve months to evaluate cycling efficiency, interaction with the microgrid and its functioning in real‑world conditions. Although modest in capacity, the project validates the integration of iron-air technology into existing grid infrastructure and provides operational data necessary for scaling to full‑size containers.
Ore Energy plans to ramp production quickly. Its goal is to manufacture up to 50 GWh of storage capacity per year by 2030 using a supply chain located entirely within Europe. Doing so would align with the European Union’s Critical Raw Materials Act and reduce dependence on imported lithium and cobalt.
The company emphasizes three applications: co‑locating multi‑day storage with wind farms, providing grid‑scale flexibility as fossil peaker plants retire, and supplying reliable backup power to data centres.
Why long-duration storage matters
With wind and solar penetrating deeper into European grids, fluctuations occur over several days, not just hours. Without long-duration storage to bridge these gaps, operators must either waste surplus generation or run gas‑fired plants. Taking this in consideration, the need for long-duration storage categories of 8–12 hours or more is very clear.
Ore Energy contends that its 100‑hour battery could reduce renewable curtailment by 44 % and cut system‑wide costs by 63 % in modelled European scenarios, shifting energy from windy weekends to calm weekdays and reducing the need for fossil peakers and over‑built renewables.
Long-duration storage batteries also enhance grid resilience by providing black‑start capability and operating without flammable electrolytes. Using abundant iron sourced within Europe also supports the EU’s energy sovereignty goals and – very important – reduces exposure to geopolitical risks.
Comparing battery technologies
Modern grids will likely deploy a portfolio of storage technologies tailored to different durations and applications. In the table below I compare Ore Energy’s iron-air battery with lithium‑ion, sodium‑ion, zinc hybrid, vanadium redox flow and liquid air energy storage (LAES) systems.
| Technology | Typical duration | Key materials & chemistry | Strengths | Limitations |
|---|---|---|---|---|
| Iron‑air (Ore Energy) | 100 hours | Metallic iron plates; water‑based electrolyte; oxygen from ambient air | Uses abundant, inexpensive materials; safe (no thermal runaway); multi‑day duration; claimed cost 7–10× lower than Li‑ion; European supply chain | Low energy density (requires large volume); technology unproven at commercial scale; regulatory frameworks still developing |
| Lithium‑ion (Li‑ion) | 2–8 hours | Lithium‑metal oxides or phosphates, graphite anode, organic electrolyte | High energy density and efficiency; mature supply chain; declining costs (~$139/kWh in 2023); fast response | Limited to short durations; relies on scarce materials; risk of thermal runaway; ~1,200–1,500 cycle life |
| Sodium‑ion (NFPP, Peak Energy) | 3–6 hours | Sodium‑iron pyrophosphate cathode; patented passive cooling | Abundant sodium; tolerates high temperatures; passive cooling cuts auxiliary power by >90 %; reduced degradation (≈33 % less over 20 years) | Lower energy density than Li‑ion; early‑stage technology; commercial deployment planned for late 2020s |
| Zinc hybrid cathode (Eos Energy) | 3–12 hours | Aqueous zinc‑bromide electrolyte with hybrid cathode | Non‑flammable electrolyte; mid‑duration storage; domestic supply chain; stackable modules for flexible sizing | Costs remain high; company not yet profitable; technology still scaling |
| Vanadium redox flow battery (VRFB) | 6–24 hours | Aqueous vanadium electrolyte stored in external tanks; redox couples control charge/discharge | Decouples power and energy (tanks sized independently); very long cycle life (≈14,000 cycles); safe and recyclable; can last >15 years | Low energy density; high upfront cost (projected ≈€260/kWh for 10 hours); vanadium supply concentrated; scale‑up mainly in China |
| Liquid air energy storage (LAES) | 8 hours to weeks | Compresses and liquefies air; stores it at ambient pressure and later expands through turbines | Uses abundant air; scalable and locatable anywhere; durations from hours to weeks; no rare materials | Moderate round‑trip efficiency (≈50–70 %); complex cryogenic systems; high capital cost |
Evaluation of Alternative Technologies
Lithium‑ion batteries dominate short‑duration storage because they deliver high energy density and fast response. However, they become uneconomic beyond 8–12 hours, rely on scarce metals and carry fire risks.
Sodium‑ion batteries, exemplified by Peak Energy’s sodium‑iron pyrophosphate system, use abundant sodium and a passive‑cooling design that cuts auxiliary power by more than 90 %. Current prototypes provide only 3–6 hours of storage and have yet to reach commercial scale.
Zinc hybrid systems developed by Eos Energy offer 3–12 hours of storage with aqueous zinc‑bromide electrolyte and promise improved safety, but they remain early in commercial deployment.
Vanadium redox flow batteries decouple energy and power, enabling lifetimes beyond 15 years and roughly 14,000 cycles. Low energy density and high capital cost are the main barriers.
Liquid air energy storage stores electricity by liquefying air, enabling durations from hours to weeks but with moderate efficiency (50–70 %); projects like Highview Power’s planned 50 MW/300 MWh plant in Manchester are still at the demonstration stage.
Implications for European energy sovereignty
Curtailment and fossil replacement
By storing excess generation across several days and releasing it during extended lulls, iron-air batteries could cut curtailment by nearly half and allow renewables to meet demand without gas‑fired peaker plants. This multi‑day flexibility could avoid oversizing wind and solar farms and trim overall system costs by up to 63 %.
Supply chains and sustainability
Lithium, cobalt and vanadium are mined in a few regions and processed largely in China. Iron however is plentiful in Europe, allowing Ore Energy and other companies to build its supply chain domestically. The water‑based electrolyte and ambient air cathode eliminate flammable solvents, and the metallic iron can be recycled at end of life, reducing environmental impacts compared with lithium‑ion systems.
Policy and market considerations
Market structures still favour short‑term services; Yilmaz notes there is no standardised way to value long-duration storage. New revenue models – capacity payments, multi‑day tenders or contracts for difference – are needed to make projects bankable. The U.S. Department of Energy aims to cut LDES costs by 90 % by 2030; Europe lacks a similar programme, so dedicated procurement targets and incentives would accelerate deployment.
Scaling and Research of Iron-Air Batteries for Long-Duration Storage
Ore Energy aims to scale its iron-air batteries to the tens of gigawatt‑hours level, rolling out full‑size 4.2 MWh containers. Success will depend on improving manufacturing efficiency, managing humidity and air intake and verifying long‑term cycling in the Delft pilot. Crucially, the company must validate its cost claims once balance‑of‑system expenses are included.
Competitors are also advancing iron-air technology. Form Energy in the U.S. has raised hundreds of millions of dollars and plans to deploy a 1.5 MW/150 MWh pilot in Minnesota and a factory in West Virginia, which again shows the growing interest in this chemistry.
Parallel progress in vanadium flow, liquid air and emerging chemistries suggests the future grid will combine short‑duration lithium‑ion with mid‑duration sodium or zinc and multi‑day iron-air or flow batteries, each filling a different niche.
Lithium‑ion remains the workhorse for short durations, while sodium‑ion and zinc hybrid batteries offer mid‑duration alternatives. Vanadium flow and liquid air systems provide modular long-duration options with different cost and efficiency trade‑offs. Iron‑air stands out for its multi‑day discharge and potential low cost but will need to demonstrate commercial viability.
Now that Europe accelerates its energy transition, long-duration storage will be essential to integrate high shares of renewables, minimize curtailment, and replace fossil backup. For this we not only need technological innovation but also regulatory and market reforms that recognize the value of multi‑day storage. Whether iron‑air batteries emerge as a cornerstone technology will depend on how quickly they can be scaled and how effectively policymakers create pathways for their deployment.
